In a working canine gracilis muscle preparation, static exercise at 2.5% of the maximum contraction will be evaluated and blood flow through the gracilis artery will be reduced by mechanical occlusion or intra-arterial norepinephrine infusion. The effects of graded flow reduction during exercise will be evaluated on creatine phosphate, adenosine triphosphate, and glycogen, using sequential skeletal muscle biopsies. In an isolated perfused vascular smooth muscle preparation, the mechanism by which digoxin potentiates norepinephrine-induced vasoconstriction will be evaluated using selective blocking agents for calcium transport (lanthanum, verapamil, and procaine). In a rat model of congestive heart failure (aortocaval fistula), the effects of treadmill exercise on the regional circulations will be evaluated using radionuclide-tagged microspheres injected in the left ventricle. These studies are designed to more fully evaluate the effects of congestive heart failure on circulation.